In conventional satellite communications systems, transponders formerly consisted of a number of separate power amplifiers each carrying multiple signals. The operating point of each amplifier was normally set to produce an average output level substantially below the saturated output level of the amplifier in order to maintain linear operation.
However, U.S. Pat. No. 3,917,998 to Welti entitled "Butler Matrix Transponder" describes an arrangement of N coupled power amplifiers for amplifying N signal paths. The N signal paths envisaged comprise the relaying of signals from at least one ground station to N locations on the earth using an orbiting satellite. The benefit of using coupled amplifiers over using a set of N non-coupled amplifiers is that the set of non-coupled amplifiers is limited to generating a power which does not exceed the peak power capability of a single amplifier in any signal path, whereas the technique which uses coupled amplifiers allows the generation of a power equal to the sum of the powers of all the amplifiers in any signal path, provided that all signal paths do not require higher than the mean power at the same time. As a result, signals that vary both above and below a mean power level are accommodated more efficiently due to a better statistical averaging of the power demanded by the N signal paths. The matrix power amplifier of the Welti patent is for use in frequency division multiple access (FDMA) applications, and provides the facility to vary the number of FDMA carrier frequencies used in each signal path and thus correspondingly the power needed in each signal path over a wide range.
A matrix power amplifier according to the Welti patent contains a butler matrix for combining a number N of input signals to be amplified to produce N different combinations of the input signals. In addition, a set of N power amplifiers are provided so that each amplifier amplifies one of the combinations to produce N amplified signals. The matrix power amplifiers also contain a buffer matrix for combining the amplified signals to produce N outputs that are amplified versions of the original N input signals. The benefit as compared with simply amplifying the original N inputs in independent amplifiers is the ability, if instantaneously needed, to devote more than the power of a single amplifier to one of the N signal paths. In principle, the matrix power amplifier can deliver the sum of the power output of all amplifiers to a single output.
The characteristics of intermodulation generated by non-linearities in a matrix power amplifier are different than in a single amplifier. It can be shown that third order intermodulation between signals input respectively to inputs I and J of the input butler matrix appears on the output numbers (2i-j).sub.N and (2j-i).sub.N of the output butler matrix. As a first step to reducing intermodulation in a matrix power amplifier, one embodiment of the present invention provides an excess number of amplifying paths so that outputs (2i-j) or (2j-i) or their corresponding inputs are not used for desired signal outputs, but are terminated in dummy loads. Thus, third order modulation between signals i and j will not be transmitted. This requires that the number of butler matrix input and output ports M be greater than the number of signals to be amplified N, wherein the remaining M-N signals are terminated in dummy loads.
It is easy to see that if only two signals are to be amplified, then using ports 1 and 2 as inputs and outputs will result in third order intermodulation appearing on ports 0 and 3, which are terminated. It is not so obvious how to achieve this when many signals are present. This problem is however solved by Babcock in another context. Babcock wanted to find a method of allocating frequency channels on an equally spaced grid to signals amplified by the same non-linear amplifiers such that third order intermodulation between any two or three signals would not fall in a channel used by a signal. The mathematical formulation of the problem is the same as for the inventive matrix power amplifier, wherein a set of integers is found I1, I2, I3 . . . such that Ii+Ik-Ij is not in the set. The solution is called "Babcock spacing". Babcock applied these integers to choosing among M frequency channels for the transmission of signals. However, the present invention applies Babcocks integer sets to choosing among M physical output channels which are used for N desired signals. Consequently, the first improvement over the prior art matrix power amplifier according to the present invention is to employ a larger matrix than the number of signals to be amplified, and to allocate input and outputs to signals or not according to the Babcock spacing or other optimum allocation, thus insuring that intermodulation emerges principally from outputs that are not allocated to signals.
Furthermore, In the present cellular communications market, there is an emphasis on making mobile telephones small, handheld units which operate from attached, rechargeable batteries. A parameter of great interest to the user of such phones is the length of time that can be spent in conversation without needing to change or recharge the battery. This parameter is simply known as the "talk-time" offered by different types of mobile phones. Of course, it is possible to offer a longer talk-time by using bigger batteries, but the bigger batteries increase the size and weight of the mobile telephone. Therefore, designers and inventors strive to construct devices which achieve longer talk-times for a given battery capacity. During a conversation, the radio transmitter power amplifier is the dominant power consumer. The efficiency of the amplifier's conversion of battery energy into radio energy thus has a direct bearing on the length of available talk-time for a cellular mobile telephone.
Cellular telephone systems were first introduced using analog frequency modulation to impress a voice on a radio signal. Analog frequency modulation has the advantage of producing a constant amplitude signal whose phase angle changes. The most efficient transmit power amplifiers can be constructed for constant amplitude signals which operate in a saturated output mode.
Original cellular systems using analog frequency modulation were also duplex, meaning that they received a signal in the reverse direction at the same time as transmitting a signal. A device known as a duplexer was therefore needed to couple both the transmitter and the receiver to the same antenna so as to avoid interference. As shown in FIG. 1(a), the antenna 12 is connected to the duplexer 16, which is formed by the filters 11 and 13. The duplexer 16 then controls the signals to and from the power amplifier 10 and the receiver 14 so as to avoid interference. FIG. 1(b) illustrates the addition of an isolator 15 in the transmit path which is used in some cases to protect the transmitter against antenna mismatches and/or to protect the transmitter from other signals picked up on the antenna and fed back to the transmitter causing an undesired phenomenon known as back-intermodulation. The isolator 15 diverts signals reflected from the antenna mismatches or received from other sources into a dummy load 18. The prior art does not disclose recycling of the energy diverted to the dummy load in order to increase the talk-time of handheld radios. A transmitter power amplifier illustrated in FIG. 1(a) can be a single power amplifier.
A transmitter power amplifier can also be constructed by combining two similar, smaller sized amplifiers. If the amplifier devices are driven in antiphase and their outputs are combined with a 180 degree relative phase so that their outputs add constructively, the amplifier is known as a push-pull amplifier. Sometimes, two similar amplifiers 20 and 21 can be driven 90 degrees out of phase and their outputs combined using a 90 degree or quadrature coupler as illustrated in FIG. 2. The quadrature coupler 23 can be formed by running two strip transmission lines in parallel proximity to each other. The energy is transferred between the lines in such a manner that a signal flowing from left to right on one line induces a signal flowing from right to left on the other line but with a 90 degree phase shift. Thus, two amplifiers connected respectively to the left hand end of a first line and the right hand end of a second line will produce signals travelling from left to right on the first line and from right to left on the second line.
When the amplifiers are approximately driven 90 degrees out of phase, the net signal flowing on the first line becomes a sum signal and the net signal flowing on the second line is a difference signal, which can be arranged to be zero. The output of the difference line is usually terminated in a dummy load 24 which normally dissipates no power. Practical tolerances of the matching between the amplifiers, the accuracy of the phase shift, energy at harmonic frequencies or antenna mismatch at the sum line output can however result in significant energy dissipation in this dummy load. The prior art does not disclose recycling this otherwise wasted energy in order to increase the talk-time of a handheld radio.
Yet another configuration of a power amplifier, known as a feedforward amplifier, may be employed in some circumstances, in which linear power amplification rather than saturated power amplification of class C amplifiers is desired. In feedforward power amplifier configurations, a more or less non-linear amplifier 30 produces an output signal that is then corrected by adding an error signal, which is produced by an error amplifier 31, to the output signal using a directional coupler 32 as described above and as is shown in FIG. 3. A waste energy signal normally produced in dummy load 33 corresponds to the unwanted difference signal. The unwanted difference signal is always produced when two dissimilar signals having overlapping spectra are added together. Again, the prior art does not disclose the recycling of the waste energy produced by the difference signal in order to increase talk-time.